1. Field of the Invention
The present invention relates to a fuel supply control system for internal combustion engines and, more particularly, the present invention relates to a fuel supply control system for a multicylinder internal combustion engine having a fuel injection valve for commonly supplying fuel to a group of cylinders of the engine.
2. Description of the Prior Art
There has been proposed a multicylinder internal combustion engine having a fuel injection valve provided upstream of the throttle valve provided within the suction pipe upstream of the intake manifold thereof for commonly supplying fuel to a group of cylinders thereof. Such a multicylinder internal combustion engine requires a less number of fuel injection valves than one equipped with a plurality of fuel injection valves with one for each of the cylinders thereof, thereby reducing the cost of the engine effectively. However, when a fuel injection valve is provided for a plurality of cylinders, it is necessary to control fuel supply operation so that fuel is distributed uniformly to all the cylinders by sufficiently atomizing fuel before the fuel flows into the intake manifold.
To this end, an internal combustion engine has been proposed, which is provided with an air throttle valve having a valve element provided at a peripheral edge thereof with a notched opening which serves as a throttle opening, and disposed so that the notched opening is positioned opposite the nozzle of a fuel injection valve provided upstream of a throttle valve with respect to the direction of flow of intake air when the valve element is shut. The engine is also provided with a Venturi tube, which is similar to that of a carburetor, disposed between the throttle valve and the fuel injection valve to regulate the degree of opening of the air throttle valve according to the negative pressure prevailing within the Venturi tube. The air throttle valve opens when the nagative pressure increases as the flow speed of air flowing through the Venturi tube increases, and shuts when the negative pressure decreases as the flow speed of air flowing through the Venturi tube decreases. Thus, the flow speed of intake air in the vicinity of the nozzle of the fuel injection valve is increased by the air throttle valve to atomize fuel satisfactorily during the low-load low-speed operation of the engine.
In such a proposed internal combustion engine, however, the flow speed of intake air drops immediately after the air throttle valve has been opened to deteriorate the atomization of fuel. Consequently, the amount of fuel that wets the throttle valve and the associated parts increases reducing the actual amount of fuel supplied to the cylinders. Furthermore, the intake air supply rate increases sharply when the air throttle valve opens, and thereby the air-fuel ratio increases temporarily, so that a lean air-fuel mixture is supplied to the cylinders.